1. Field of the Invention
The invention relates to electromagnetic radiation detectors, and more particularly, to pattern-sensitive detectors operating in the extreme ultraviolet (EUV) light range.
2. Description of Related Art
The detection of radiation in the extreme ultraviolet range (EUV) is conventionally accomplished using EUV-sensitive photodiodes which have specialized structures allowing the detection of 60 eV-500 eV photons through the use of shallow (.about.300 .ANG.) N-P junction diffusion. The use of silicon-based photodiodes to detect EUV radiation is complicated by the fact that such photodiodes can detect photon energies as low as the far infrared with better sensitivities than in the EUV. This is a consequence of the higher transparency of the silicon material of the N-P junction to light at wavelengths near the infrared as compared to light at wavelengths near the EUV. To overcome the limitations of the EUV-silicon interactions, shallow junction photodiodes have been developed. A shallow junction is necessary since photons in the EUV spectrum are highly attenuated in most materials and it is necessary for photons to enter the depletion region of the N-P junction in order to be adequately detected. Prior art photodiodes are discussed in the articles "Stable Silicon Photodiodes For Absolute Intensity Measurements In The VUV and Soft X-Ray Regions," by E. M. Gullikson, et al., (Journal of Electron Spectroscopy and Related Phenomena, Proc. of the VUV-11 Conference), "Photoemission From Silicon Photodiodes and Induced Changes in the Detection Efficiency in the Far Ultraviolet," by R. E. Vest, et al. (Jun. 20, 1997), and "Normal Incidence Spetrophotometer With High-Density Transmission Grating Technology and High-Efficiency Silicon Photodiodes for Absolute Measurements," by Ogawa et al.(Optical Engineering 32(12), 3121-3125, December 1993).
A recognized problem with shallow junction photodiode structures of the prior art is that optically generated carriers are intercepted from the detector's electrodes by recombination with surface impurities. This problem usually prevents the detection of the intercepted photons and results in an inconsistent current response to incident radiation. According to tests, contaminants absorbed through the light-impinged surface of the photodiode can be responsible for as much as a thirty-percent variation in the response characteristics of the device.
Use of shallow junction silicon photodiodes in aerial image measurement (AIM) and lithographic and other applications in the EUV light range requires the arrangement of a precisely defined and positioned absorber having appropriate patterning formed thereon for selective transmission of the EUV light. Current EUV lithography systems operate at a wavelength of 13.4 nm. The short absorption depth of 13.4 nm radiation in all materials dictates the use of extremely thin membranes to support the absorber. Moreover, these membranes must be suspended at a precisely controlled distance from the photodiode while being subjected to the abusive evacuation processes required for transmission of the EUV radiation (air is impermeable to EUV radiation). Rupture of the membranes is not uncommon, providing for low yield and costly assembly and operational procedures.